Headphone Jack incredibly quiet

I've had my MBP for 3 years, and I'm usually decent enough at fixing minor problems that I have. I got a new pair of Sennheiser HD 280 Pro's today, and I can't use them with my computer for some reason.
I've attempted to reset the pram, I've checked all the system preferences, I actually CAN hear the audio through the phones. The problem is that it is VERY faint. It's barely audible at max volume. I've never had this problem before...
I've tried to plug a different pair of headphones in and I have the same problem.
So basically, I'm stuck troubleshooting, and wondering what I can do to remedy this. Any help would be greatly appreciated.

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